1. Field of the Invention
The present invention relates to image heating devices for heating recording materials that carry images, and in particular, relates to those used as heat fixing devices for heating recording materials having unfixed images (toner images) formed thereon in image forming apparatuses such as printers that form images on recording materials using image forming processes such as electrophotographic processes.
2. Description of the Related Art
Among various fixing devices, which are examples of image heating devices, in practical use, those of the heating roller type are the most popular. Fixing devices of this type include a fixing nip formed of a fixing roller and a pressurizing roller. The fixing devices fix toner images on recording materials that carry the toner images by applying heat and pressure to the recording materials using the fixing nip. The fixing roller and the pressurizing roller are pressed into contact with each other by a pressure-applying mechanism. Moreover, some of these fixing devices include a depressurizing mechanism for removing or reducing the pressure applied to both rollers by the pressure-applying mechanism in order to easily remove recording materials jammed in the fixing nip, or in order to regulate the deformation of rubber layers of the fixing roller and the pressurizing roller caused by the pressure-contact state continued over a long period of time. In addition, some of these fixing devices include a mechanism for changing the pressure applied to both rollers by the pressure-applying mechanism in order to ensure the optimum fixability of the toner images.
Pressure-changing mechanisms (hereinafter referred to as mechanisms having at least one of a depressurizing function and a pressure-changing function) of the most popular type include cam members. In general, a pressure-applying mechanism that applies pressure between the fixing roller and the pressurizing roller uses springs or the like disposed at either end of the rollers, and thus each of the cam members also needs to be disposed adjacent to either end of the rollers. The pressure applied between the rollers by this pressure-applying mechanism can be changed by rotating the cam members acting on the pressure-applying mechanism manually or using motor power. For example, those disclosed in Japanese Patent Laid-Open Nos. 2-157756, 11-125985, and 2000-29347 rotate the cam members using motor power.
In general, the pressure-changing mechanisms using motor power as disclosed in Japanese Patent Laid-Open Nos. 11-125985 and 2000-29347 rotate two cam members by transmitting the motor power to one end of the rollers. In this case, a dedicated shaft for supporting the two cam members is required. Moreover, this shaft requires a certain degree of torsional rigidity such that the rotational phases of the two cam members do not vary widely. This leads to an increase in production costs, and also leads to an increase in the size of the device for ensuring the space for the dedicated shaft.